Electrical conductors provided with an electrical insulation in the form of a varnish filled with particles of chromium oxide (Cr2O3) or iron oxide (Fe2O3) are known to provide excellent resistance against partial discharges. These types of insulated electrical conductors are typically used in electrical motors and are then referred to as enameled wires.
EP 356 929 A1, hereby incorporated by reference, gives an example of an enameled wire that has an insulating coating system comprising a basecoat and a topcoat. The topcoat comprises an organic polymer of, for example, polyesteramide (PEA) and a filler in the form of chromium oxide particles or iron oxide particles with an average particle size of 0.005-30 μm, preferably 0.15-10 μm. The topcoat is applied on a basecoat of polyester or polyesterimide (PEI) surrounding the conductor. The amount of chromium oxide particles must be high enough for forming a percolated structure. By this an intrinsic resistivity of 104-108 ohm m is achieved in the top-coat, which will electrically shield the insulation system under electrical discharges. Thereby the initiation of electrical erosion of the insulating coating is prevented. Once the top-coat is penetrated or cracked, electrical erosion proceeds rather fast through the basecoat. One disadvantage with the above described type of insulation system is that with increased requirements on the insulating coating to withstand high electrical fields a higher amount of chromium oxide is required. With this higher amount of chromium oxide the mechanical properties of the insulation system are decreased. As the coated wire is subject to bending, for example, during the manufacturing of motors, it is necessary that insulation system has excellent mechanical properties, such as excellent bending strength.
Especially for low-voltage frequency-converter controlled motors there are increased requirements on the conductor insulation coating to withstand flash-over voltages that can give rise to partial discharges in the insulation system of the conductor, and subsequently possible electrical erosion of the insulation system. There is therefore a need for an electrical insulation system for metallic conductors, such as wires, that have a similar or higher resistance against partial discharges than insulation systems of today.